The Fisher Body Unit Number is unique to Fisher Body meaning it has nothing
to do the car's final VIN and will not match the last six digits of
the car's VIN.

Unlike the VIN that increases by 1 for each car regardless of the series
or body style the body number appears to be fairly sequential based on the
style number. This means that any two Chevelles built one after the other
would have consecutive VINs such as 101000 and 101001 but the body numbers
would not be consecutive unless they were identical styles (and even then
it'd be a rare occurrence due to other issues at play). Another way to explain
it is for any given time of the year there is bound to be more of one particular
style built than another so the more popular style (to that point in time)
would have a higher body number than the lesser popular style. For example
a Malibu SS sport coupe (style 13837) built the first week of January would
have a higher body number than a 300 Deluxe 2-door sedan (13411) built
in February simply because more Malibu SS sport coupes were being built
than 300 Deluxe 2-door sedans.